Fletcher Munson 'Loudness Control' Video

As you turn up the volume, a system with de emphasized midrange develops a smiley face response just like one with boosted bass and treble.

@lewm 

his method is related to cars, and avoiding distortion from car amps. At home, if you have the headroom, which you ought to, it is for low level listening, I like an automatically and progressive implementation like my Yamaha receiver and my Chase RLC-1. 

Even then, you need to set up your 'normal' volume 1st, then use the Loudness control to reduce volume.

My old Acoustic Research amplifier has tone controls modeled off the Fletcher-Munson curves and notably uses sliding inflection points for the equalization -- the more boost was dialed in, the closer the inflection hinge point came to the center.  This was a sensible design that was lauded at the time.

Dr. Floyd Toole covers this subject thoroughly in his book "Sound Enginnering."  One thing he discusses is that experienced listeners are much less likely to boost the bass and treble from "ideal" levels than are novices.  Also, everyone seems to like bass so if any tweaking is to occur even among experienced listeners it is likely to be in the bass region.  In all of reported experiments under controlled conditions novices adjusted both bass and treble to quite high levels.  This from the 4th edition, chapter 12 if memory serves.

Yamaha had it right a long time ago. My 1978 CR2020 had a "Loudness" control knob.